In equipment for handling sheet materials, such as plastic films in bag making machines, it is advantageous to provide registration indicators upon the material or film for purposes of sealing and perforating the film at selected locations. In particular, registration marks may be provided on film which allow the machine to seal or perforate the film with respect to the location of the registration mark. For example, in a bag making process, a perforation is provided in relation to a seal such that a continuous film including seals and perforations may be separated to provide individual bags. In the past, the perforation was mechanically registered to the seal such that the perforation was made in the proximity of the seal. While this arrangement provided satisfactory results, transient stretching in the film, film speed variations within a machine and operation of preceding process steps within the machine or in preceding machines resulted in distances between the seal and perforation which were not consistent. For example, the mechanical registration system may be set to provide a 1/4 inch space between the seal and perforation, but due to film stretching, variations of film speed in the machine, or effects of preceding processes, the distance between the seal and perforation may vary within a range of 1/4 inch, e.g., between 1/8 inch and 3/8 inch. In high speed machines, such variations can quickly cause unacceptable perforation to seal misregistration.
One known arrangement for detecting printed marks, such as registration marks, on web materials involves the use of a light source aligned generally perpendicular to the web and a sensor positioned substantially colinear with the light source. However, such an arrangement is not entirely successful at detecting features in certain sheet materials, and in particular, is not well suited to detecting a seal or perforation in plastic film materials.
Accordingly, it would be desirable to provide the capability to detect a seal in a plastic film and control film perforation such that perforations are directly registered with respect to the seal. However, in the past, it has been difficult to detect the presence of a seal in a moving film accurately and consistently. Thus, it would be desirable to provide a seal detection method and arrangement which can accurately and consistently detect a seal in a moving film, even where the film is moving at linear speeds in excess of 600 feet per minute. Additionally, it may be desirable to detect a perforation for properly separating bags being removed from a roll of bags and/or folded.